The present invention relates to an optical device which has a mirror, support beams, an actuator and optical waveguides formed by semiconductor anisotropic etching or the like on a substrate and in which the mirror is pushed into and pulled out of the optical path between the waveguides to switch the optical path or control the quantity of light to be coupled to each incidence port.
FIG. 1 shows, as a prior art example of an optical device of this kind, the configuration of a MEMS (Micro Electro Mechanical Systems) conventional optical switch described in International Publication Number WO 01/11411 A1 (published Feb. 15, 2001, hereinafter referred to as document 1).
In the top surface 100u of a plate-like flat substrate 100 (which is shown divided into two for ease of illustration but has a one-piece structure in practice) there are formed crosswise four waveguide (fiber) channels 105A to 105D coupled at one end together. An area defined by the fiber channels 105A and 105B perpendicular to each other is used as an actuator forming area 101.
In the actuator forming area 101 there is formed a trench or slot 112 cut in the substrate surface 100u at an angle of 45° to each of the fiber channels 105A and 105B. Disposed in the slot 112 is a movable rod 116. The movable rod 116 has coupled thereto on both sides support frames 134A and 134B at one end thereof, and the other ends of the support frames 134A and 134B are fixed to fixed support structures 130A and 130B via leaf-spring-like support beams 124A and 124B, respectively. Thus the movable rod 116 is supported in a manner to be movable lengthwise thereof in parallel to the substrate surface 100u. 
Four optical fibers 106A to 106D are disposed in the fiber channels 105A to 105D, respectively. The movable rod 116 supports at one end thereof a mirror 102 to hold it initially in the intersection 105c of the optical axes of the optical fibers 106A to 106D extending radially therefrom, and the movable rod 116 couples at the other end to a comb-type electrostatic actuator 122.
The comb-type electrostatic actuator 122 is composed of a movable comb electrode 110 and a stationary comb electrode 108 interleaved with each other, disposed in the longitudinal direction of the movable rod 116 but extending in a direction perpendicular thereto. The movable comb electrode 110 is fixed to the movable rod 116, and the stationary comb electrode 108 is fixed to the bottom of a recess 115 formed in the surface 100u of the actuator forming area 101.
The recess 115 communicates with the slot 112, and the movable rod 116 extends into the recess 115. In the recess 115 there are disposed the comb-type electrostatic actuator 122, the support beams 124A, 124B, the support frames 134A, 134B, and the fixed support structures 130A, 130B. As is the case with the stationary comb electrode 108, the fixed support structures 130A and 1301B are fixed to the bottom of the recess 115, but the other structures, except the stationary comb electrode 108 and the fixed support structures 130A, 130B, are held above the bottom of the recess 115.
Voltage application across the movable and stationary comb electrodes 110 and 108 creates an electrostatic attractive force, which moves the movable rod 116 toward the stationary comb electrode 108 to pull the mirror 102 out of the intersection 105c. Upon stopping the voltage application, the movable rod 116 moves back toward the intersection 105c by elastic restoring force of the support beams 124A and 124B, bringing the mirror 102 back to its extended position in the intersection 105c. 
With the mirror 102 pushed into the intersection 105c, light emitted from the optical fiber 106A, for instance, reflects off the mirror 102 for incidence on the optical fiber 106C, and light emitted from the optical fiber 106B reflects off the mirror 102 for incidence on the optical fiber 106D. With the mirror 102 pulled out of the intersection 105c, the light emitted from the optical fiber 106A is incident on the optical fiber 106D, and the light emitted from the optical fiber 106B is incident on the optical fiber 106C.
In the conventional optical device of the above-described configuration, the positional relationship between the point of application of driving force by the actuator to the movable portion and the points of supporting it by the support beams is not optimum in terms of dynamic characteristics of the movable portions.
Accordingly, when the driving force by the actuator contains a vector component in a direction different from a desired direction in which to drive the movable portion, there is the fear of pitching, rolling, or yawing of the movable portion.
In the conventional optical switch of FIG. 1, the position of the comb-type electrostatic actuator 122 relative to the movable portion including the mirror 102 and the movable rod 116 is disposed the other end of the movable rod 116 in opposing relation to the mirror 102 with the support beams 124A and 124B interposed between them. Now, let it be assumed that the comb-type electrostatic actuator 122, though just completed, is structurally nonuniform or asymmetrical in that the gaps g between sidewalls of adjacent comb fingers of the movable and stationary comb electrodes 110 and 108 interleaved with each other, which ought to be uniform, differ on the right and left of an arbitrary one of the adjacent comb fingers. Letting three orthogonal axes X, Y and Z be defined as shown in FIG. 1, and letting the direction of arrangement of the comb fingers and the longitudinal direction of the movable rod 116 be represented by X and Y, respectively, the driving force by the comb-type electrostatic actuator 122 contains a vector component of the X direction. The reason for this is that the electrostatic attractive force acting between comb fingers of different potentials is in inverse proportion to the width of the gap g between adjacent comb fingers, creating a larger attracting force between closely spaced-apart comb fingers with a small gap therebetween.
In this instance, since the point of application of the driving force is disposed at the end of the movable rod 116 opposite to the other end thereof carrying the mirror 102 as mentioned above, moment of force is applied to the supporting points of the support beams 124A and 124B, causing the movable portion including the movable comb electrode 110 to yaw about the Z axis. If this phenomenon becomes excessive, the movable and stationary comb electrodes 110 and 108 contact each other, developing a short circuit. Accordingly, the optical switch of FIG. 1 is inappropriate from the viewpoint of stability of the movable portion during switching operation.